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TPS62740: characteristics EN pin

Daan Vreeken
Prodigy 30 points
Part Number: TPS62740
Other Parts Discussed in Thread: TPS62125, TPS62745

For a new product we tested the TPS78001 where the EN pin is used to switch the device on and off, in this case we make use of the hysteresis of the EN pin.

Its datasheet mentioned on page4 that the input has logic levels, but on page11 of its datasheet (figure 35) also the real levels are specified as function of the temperature.

In our application  the input supply voltage of the TPS78001 is fed to the EN pin via high-ohmic resistors, which works very well.

Now we consider to use the TPS62740 for better efficiency, it seems to have similar input characteristics on the EN pin, but its datasheet is only mentioning the logic levels.

Can you confirm/specify the actual hysteresis levels also for the TPS62740?

  • 0
    TI__Guru**** 150650 points
    The EN pins on these devices work similarly. They are logic inputs with some hysteresis.

    Yes, this DC/DC has lower Iq as well compared to the LDO, and will thus give better efficiency.

    It's not clear why you have high-Ohmic resistors on EN. EN just needs to be tied high or low. Are you creating a voltage divider with 2 resistors on EN?
  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Thanks for the quick responce.

    Indeed we use a high ohmic restistor devider to monitor the input voltage of the regulator. Reason is the following: the average current of the circuits can easily be delivered by our supply (a small solar panel), but during startup, and a short periode after that, the peak currents have to be supplied by a buffer capacitor.

    If we would start via the internal undervoltage detector at 2.075V the circuit already stops when the capacitor is discharged to 1/925V, so we may only have 0.225V decay of the capacitor.  

    To make sure that we can safely start we monitor the capacitor voltge with the EN pin via a high ohmic devider (1M / 270k), the circuit starts now around 3.6V an should stop just above the threshold voltage of the  internal UVdetector. This allows for a more practical 1.6V decay of the capacitor voltage.

    It would help us enormously if you could guarantee the spec. of the EN pin of the TPS62740 similar to the spec of the TPS78001. (preferrably by including it in the datasheet).

  • 0 in reply to Daan Vreeken
    TI__Guru**** 150650 points
    Thank you for explaining. I think this is the same challenge as addressed by this app note: www.ti.com/.../slyt488.pdf Note that the TPS62125 has a precise EN pin spec, which is what enables your circuit to work as you described. If your system currents are not terribly low, you might want to consider the TPS62125 and use it in the same way. It has higher Iq than the TPS62740, but its EN pin is designed to do exactly what you need. An external SVS controlling the EN pin of the TPS62740 is another possibility.

    The TPS62740 and TPS78001 have exactly the same type of specification for the EN pin. This is what is found on page 5 of the TPS62740 D/S.

    The figure you refer to in the LDO D/S shows the typical hysteresis and variation over temp of the threshold voltage. Figures like this are usually a sample waveform from 1 unit and do not imply a specification/guarantee. Such a digital input pin usually has large possible variations over temperature and over the IC process (different ICs from different wafers could show a large difference). This is not a robust way to operate the circuit.

    Finally, it might be possible for the TPS62740 to work well in your application, if the rise time of the input voltage is somewhat fast. As specified in the D/S, the TPS62740 has a rather long turn on delay time (10 msec typical). So, if your input voltage rises faster than this, it will be fully up when the TPS62740 turns on and starts drawing current.

    While likely unrelated, here is another concept to work with solar panels: www.ti.com/.../slyt478.pdf

    Lastly, the TPS62745 is a higher input voltage version of the TPS62740, in case you want to add more solar cells in series.